I have an itty bitty baby betta in a 10 gallon all by himself. He seems very happy and healthy. Eats well.
The smallest tank we have is a 3.5 gallon with my daughter's fish in it. All our tanks have heaters and most have filters.

Her advice really just comes from being misinformed. They don't live in tiny puddles. Even if they did, you really can't compare that to the fish we tend to own. The wild bettas don't have the long flowing fins that many people find so attractive.

I have an itty bitty baby betta in a 10 gallon all by himself. He seems very happy and healthy. Eats well.
The smallest tank we have is a 3.5 gallon with my daughter's fish in it. All our tanks have heaters and most have filters.

Her advice really just comes from being misinformed. They don't live in tiny puddles. Even if they did, you really can't compare that to the fish we tend to own. The wild bettas don't have the long flowing fins that many people find so attractive.

Actually, the long finned bettas generally need less space than plakats because their long fins cause them to work a bit harder to swim. So they don't need to swim around as much to get enough exercise.

For some it is. I judge based on the fish.
I tried to move my Egil to a smaller tank. He ate his tail. I tried moving him to a bigger tank. He completely destroyed his tail. Put him back with his brothers and now he's fine. -_- Still nibbles his tail from time to time though. I wouldn't dare give my Odin more or less space. His vision is too poor.
I would feel bad putting Hannibal in anything less than 3.5 gallons. He has a body that is over 2 inches long with at least 3 inches of fins. Cranky little thing.

For some it is. I judge based on the fish.
I tried to move my Egil to a smaller tank. He ate his tail. I tried moving him to a bigger tank. He completely destroyed his tail. Put him back with his brothers and now he's fine. -_- Still nibbles his tail from time to time though. I wouldn't dare give my Odin more or less space. His vision is too poor.
I would feel bad putting Hannibal in anything less than 3.5 gallons. He has a body that is over 2 inches long with at least 3 inches of fins. Cranky little thing.

Do Egil and Odin have about 3.3gallons each? (10gal divided 3 ways).

I personally wouldn't put a regular sized betta in less than 2.5 gallons, and I wouldn't put a giant or king in less than 5. But for those that keep their betta in one gallons, I think it's perfectly fine if you care for them properly...